Many optical devices must be inspected and calibrated to determine their performance over the entire viewing area. For example, a focal plane array includes thousands of pixels. When illuminated, each pixel produces a respective electrical signal value related to the intensity of incident light. The electrical signal values of the individual pixels are supplied to a processor, which assembles the information into a viewable image or otherwise processes the pixel information.
Ideally, all of the pixels of the focal plane array would respond to light in exactly the same way. In reality, however, the microelectronic fabrication techniques that produce the semiconductor pixel structures have sufficient variability that the pixels do not respond to light in exactly the same way. When the focal plane array is calibrated, the threshold and electrical signal response as a function of intensity for the pixels are recorded and supplied to the processor as a calibration standard. This information, together with the electrical signal produced during service, permit a calculation of the actual incident light intensity as a function of the electrical signal output of each pixel.
To perform the calibration, it is important to illuminate the entire local plane array with a uniform intensity of light, so that the different responses of the nonuniform pixels may be evaluated and calibrated. Although a number of illumination sources are known and in use, over the wide angle required to illuminate a large focal plane array such illumination sources are uniform only to about 95-98 percent. A higher degree of uniformity is desired in order to achieve a better calibration of the pixels of the focal plane array.
Similar problems arise in calibrating and evaluating other types of optical devices, due to the unavailability of large-angle, uniform radiance (i.e., illumination) sources.
There is a need for such a highly uniform radiance source that is uniform over a large angle and thence a large illuminated area. The present invention fulfills this need, and further provides related advantages.